From U.S. Pat. No. 6,763,609, issued to Su on Jul. 20, 2004: “Conventional footwear construction has generally followed two methods, welted construction and molded construction. Welted construction generally stitches an outer sole to an upper to form footwear and thus is labor intensive and therefore expensive. Molded construction generally uses molds and liquid sole material to form the sole onto an upper. This is a less expensive alternative that results in a very durable and flexible outer sole, however the bond between the outer sole and the upper may not be as durable as welted attachment.”
The assembly of sole to upper known as “Goodyear welt” is well documented in the footwear industry. A welt is a strip of leather, rubber, or plastic that is stitched to the upper and insole of a shoe, as an attach point for the sole. The space enclosed by the welt is then filled with cork or some other filler material, usually either porous or perforated, for breathability. The outsole is both cemented and stitched to the welt.
The Goodyear welt involves a method of attaching the sole of a shoe to the upper. According to the method, the upper is drawn over the last and held temporarily whilst the welt is stitched to the upper and inner sole. The sole is hand-stitched through the welt to complete the process. This construction allows multiple sole replacements, extending the life of the footwear.
The welt generally includes a horizontal base from which upward and downward ribs extend. The upwardly extending rib is stitched to the upper and the downwardly extending rib or the base is stitched to the sole to hold the upper and outsole together. The welt itself is semi-rigid or hard to withstand the forces exerted through the stitching. In the finished shoe, the welt provides an aesthetically pleasing transition between the sole and upper.
Goodyear welt constructed work boots are known to be very strong and durable. This kind of construction is a synonym of toughness in the mind of wearers because of the durability of the assembly of sole to upper. Goodyear welt construction is a shoemaking process that has proven itself to be long-lasting under heavy conditions. This is why the particular and distinctive stitching of the Goodyear welt, bonding together the welt and sole, has represented, for years, durability and toughness. The Goodyear welt stitching has become, with time, a proof of quality and good craftsmanship in the business of work and safety footwear.
Although the welt construction is durable and aesthetically pleasing, the welt and associated stitching compromises the flexibility and waterproof properties of the shoe. This is true in welt constructions where the sole is a hollow shell filled with a flexible polyurethane-foam material. Furthermore, the inclusion of the welt in welt construction footwear increases the cost of and the time required in manufacturing such footwear.
The cost and complexity of manufacturing Goodyear welt boots influence the search for a more efficient shoe-making process, following the popular industrial injection molding process. The assembly of sole to upper called “injection” is also called “direct-attach” construction because it does not require a welt of upper to sole or gluing. In this process, the sole is directly attached to upper by molding it in a mold at high temperature.
Consequently, injected footwear is quite flexible and thus comfortable when compared to other types of constructions. Moreover, injection of soles ensures a strong fiber bonding and seal between sole and upper. This makes waterproofing of footwear much easier to achieve. Injection is an efficient and cost-effective process that contributes to keeping the price of footwear relatively low.
More particularly, injection of midsoles with a low-density polymer gives the boot a soft cushioning, keeping a good memory of shape. Injected midsoles are, thus, more comfortable over a long period of time. In recent years, double-injected work boots have become very popular because of their lightness, flexibility, comfort and all the benefits associated with the quality of bonding (waterproof benefit, for example). However, for many reasons (e.g., because of their plastic-type look, because of the absence of a Goodyear stitch line), injected work boots do not have an image of durability and toughness.
In order to simulate the look of the Goodyear welt on shoes and boots, soles have been molded with a stitch line pattern (also molded). However, from a close stand point, it is clear that the stitch line is molded. In another embodiment, a stitch line is typically added to a molded sole, with the sole and stitch line assembly being subsequently glued to the upper. One of the issues with such a boot/shoe construction is that it may allow water infiltration between the upper and the sole. Moreover, the bonding of the molded sole with the upper is not as strong as with a direct-attach construction.